A CRT television creates images by firing a beam of electrons at a glass screen coated in phosphor, a material that glows when struck. This basic process, repeated millions of times per second, produces the bright, vivid picture that defined television for over half a century. Every part of the TV, from the vacuum inside the tube to the magnets around its neck, exists to control that beam with extreme precision.
The Electron Gun: Where It All Starts
At the narrow rear end of the tube sits the electron gun. Its core component is a cathode, a small metal element made from a tungsten-barium alloy chosen for its ability to release electrons easily. When the TV powers on, this cathode heats to between 1,000 and 1,400°C. At that temperature, electrons essentially boil off the surface in a process called thermionic emission, the same principle behind old vacuum tube radios.
Once freed, the electrons pass through a series of metal grids and focusing elements that shape them into a tight, narrow beam. A control grid regulates how many electrons pass through at any moment, which determines how bright a given point on the screen will be. More electrons means a brighter dot; fewer electrons means a dimmer one. The beam then accelerates toward the screen, pulled forward by a powerful positive voltage at the front of the tube, typically around 25,000 to 30,000 volts.
Steering the Beam With Magnets
A stationary beam would only light up one dot in the center of the screen. To paint a full image, the beam needs to sweep back and forth across the entire screen surface hundreds of times per frame. This job falls to the deflection yoke, a set of electromagnetic coils wrapped around the narrow neck of the tube just ahead of the electron gun.
The yoke contains two pairs of coils. One pair handles horizontal deflection, sweeping the beam from left to right. The other handles vertical deflection, pulling the beam from top to bottom. By varying the electrical current flowing through these coils, the TV modulates the strength of the magnetic fields. A moving electron passing through a magnetic field experiences a force that pushes it sideways, perpendicular to both its direction of travel and the field itself. Adjusting the current precisely controls where the beam lands on the screen at any given instant.
How the Beam Paints a Picture
The beam doesn’t illuminate the entire screen at once. It traces the image line by line, starting at the top-left corner and sweeping horizontally to the right, then snapping back to start the next line slightly lower. In the North American NTSC standard, each complete frame contains about 525 lines displayed at roughly 30 frames per second.
To reduce flicker without doubling the required bandwidth, CRTs use a trick called interlaced scanning. Instead of drawing every line in sequence, the beam first paints all the odd-numbered lines (field one), then returns to the top and paints all the even-numbered lines (field two). Each field takes about 1/60th of a second, so your eye sees 60 screen refreshes per second even though only 30 complete frames are drawn. The European PAL standard uses a similar approach at 25 frames and 50 fields per second.
Phosphors: Turning Electrons Into Light
The front face of the tube is coated on its inner surface with phosphor compounds, materials that emit visible light when hit by high-energy electrons. In a black-and-white TV, a single uniform phosphor layer covers the screen. Color TVs use three different phosphor types arranged in precise patterns: one that glows red, one green, and one blue. The red phosphor is typically a europium-doped compound, the green uses terbium-doped materials, and the blue relies on cerium-doped compounds. By varying the intensity of each color at every point, the TV mixes them to produce the full spectrum of visible color.
Phosphors don’t glow forever after being struck. Their light fades quickly, a property called persistence. The phosphors in standard TV tubes have a medium decay time, long enough to remain visible until the beam sweeps past again on the next field, but short enough to avoid ghosting or smearing on fast-moving images.
Shadow Masks and Aperture Grilles
A color CRT actually contains three electron guns, one for each color. The challenge is making sure each gun’s beam only hits its matching phosphor dots and doesn’t accidentally light up the wrong color. This is solved by a thin metal layer positioned just behind the phosphor screen.
The most common design is the shadow mask, a sheet of metal perforated with hundreds of thousands of tiny holes arranged in a triangular (triad) pattern. Each hole is angled so that the red gun’s beam passes through and hits only red phosphor, the green beam hits only green, and the blue beam hits only blue. The downside is that the mask blocks a large percentage of electrons, around 85% in triad designs, which means the screen needs more power to achieve a bright image. Shadow masks can also become magnetized over time, causing color distortion.
Sony’s Trinitron televisions used an alternative called an aperture grille: thin vertical wires stretched taut from top to bottom instead of a perforated sheet. Because wires block less of the beam (under 25%), aperture grille sets were noticeably brighter, produced richer colors, and looked sharper. The tradeoff was weight, cost, and the need for one or two thin horizontal stabilizing wires to keep the vertical wires from vibrating. Those stabilizing wires were sometimes visible as faint lines on bright images.
The Vacuum Inside the Tube
None of this works without a near-perfect vacuum. If air molecules remained inside the tube, electrons would collide with them and scatter before reaching the screen. The interior of a CRT is evacuated to less than one millionth of atmospheric pressure, sometimes far lower, achieved by baking the sealed tube in an oven at 375 to 475°C to drive out residual gas.
That vacuum creates a real structural hazard. With normal atmospheric pressure pushing inward on a large glass surface and almost nothing pushing back, the tube is under enormous stress. If the glass cracks, the tube can implode violently, sending shards inward and then outward. To prevent this, manufacturers bond a tensioned metal rim band around the perimeter of the screen. The band is heated during assembly so it expands, fitted over the glass, and then allowed to cool and shrink. This puts the glass under constant compression, strengthening it and allowing the faceplate to be thinner and lighter than it would otherwise need to be.
High Voltage and X-Ray Shielding
The 25,000 to 30,000 volts needed to accelerate electrons to useful speeds create a secondary concern: when fast-moving electrons slam into the phosphor screen or the shadow mask, they can generate low-level X-rays. At 30 kV, the energy is high enough to produce soft X-ray photons. CRT manufacturers addressed this by incorporating lead into the glass itself, particularly in the funnel section that connects the neck to the faceplate. CRT glass contains 14 to 23% lead by weight, which absorbs the X-rays before they can reach the viewer. This lead content is also the reason CRT disposal is an environmental concern, as the lead can leach into groundwater if the glass ends up in a landfill.
Degaussing: Fixing Color Problems
Because the shadow mask is made of metal, it can pick up residual magnetism from nearby speakers, magnets, or even the Earth’s magnetic field after the TV is moved. This magnetism bends the electron beams slightly off course, causing patches of incorrect color on the screen.
Most color CRTs include a built-in degaussing coil, a loop of wire mounted behind the screen. When the TV is first turned on, this coil briefly generates a strong alternating magnetic field that rapidly fades to zero. The decaying field neutralizes any stray magnetism that has accumulated on the mask. If the built-in coil isn’t strong enough, an external handheld degaussing wand can be passed slowly across the screen to clear more stubborn distortion.
Why CRTs Still Matter for Gaming
CRT televisions process incoming video signals with virtually zero added delay. The electron beam begins drawing the image the moment the signal arrives, so the top of the screen updates almost instantly. At 60 Hz, the middle of the screen is only about 8 milliseconds behind the signal, and even the bottom of the screen is no more than 16.67 milliseconds behind. There’s essentially no internal processing lag, a stark contrast to LCD panels, which must receive an entire frame, process it, and then update their pixels. Even fast modern LCDs add around 8 milliseconds of display lag at 60 Hz, and many budget sets add considerably more.
This near-instantaneous response is why competitive retro gamers and fighting game enthusiasts still seek out CRTs. For older consoles that output 240p signals, CRTs also display the image exactly as the developers intended, with natural scanlines and smooth motion that flat panels can only approximate through scaling and post-processing.